Within the process gas industry, and in particular within the air separation market, the continuing increasing costs of electric power makes the energy efficiency of gas separation systems of ever increasing importance. The ability of a gas separation system, and an air separation system in particular, to deliver a consistent and uninterruptible high purity oxygen product at all times and under all annual site ambient temperature operating conditions can be of equal importance.
Gas separation feedstock gases can comprise low positive pressure ‘vapor recovery’ gases developed within facility process operations, or in particular feedstock conditioned atmospheric air at slight sub-atmospheric pressure to produce a predominant rich oxygen product gas. In the case of some production process vapor recovery gases, the desired product gas can be the major molecular gas component within the feedstock gaseous mixture, and this major molecular component can be the gas that is adsorbed within a selected molecular sieve material of correct pore size. In this process gas application case, the desired product gas would be vacuum extracted and delivered during the ‘desorption operation’ sequence and the waste gas would be extracted from the ‘adsorption operation’ sequence.
In the case with large commercial, industrial, manufacturing, or power generation facilities wherein a predominant rich oxygen mixture is required for oxy-fuel combustion systems that employ best available technology (BAT) to greatly reduce fugitive combustion exhaust emissions and to greatly increase operating fuel combustion systems' thermal efficiencies, it is important that an air separation system be market available that economically encourages and supports the employment of the BAT Oxy-fuel combustion technology with very low air separation system power consumptions. The air separation system should preferably incorporate all of the following: (a) a commercially available molecular sieve adsorbent material having acceptable nitrogen absorbency characteristics (b) greatly reduced kWh electric power consumptions per ton of oxygen product as compared to currently available art systems, (c) a preferred reduction in system installed capital costs per TPD oxygen rating, and (d) ability to deliver to a facility the required 93% to 94+% oxygen purity and full rated TPD oxygen capacity continuously during all typically site experienced climatic operating temperatures.
The common methods of separating nitrogen from air to provide a highly predominant oxygen gaseous product stream includes the utilization of (a) multi-stage membrane separation units; (b) pressure swing adsorption (PSA) units for producing moderate pressure supplies of predominant oxygen gas; (c) vacuum pressure swing adsorption (VPSA) units, or also interchangeably referred to as vacuum swing adsorption swing units (VSA) units. VPSA or VSA units produce a low positive pressure delivery of predominant oxygen product, and employ air blower/compressors to both produce a positive pressure supply of air to the system adsorber vessels and to induce a vacuum extraction of nitrogen waste gases from the adsorber vessels; (d) Cryogenic-type air separation plants for 99.999% oxygen purity delivery requirements and/or where larger oxygen TPD production rates, production online reliabilities, operating power costs, installed costs, or other factors combine to make current art VPSA or VSA systems a second choice.
However, for facilities with low pressure delivery of 90 to 92% oxygen purity gas and less than 350 TPD oxygen requirements, it is reported that single or multiple parallel-connected current art VSA systems are considered to be the most economic cost choice available for most applications.
Currently employed VSA air separation system processes for producing 85% to 92% low pressure oxygen gas mixtures of high-range double-digit to low range triple-digit TPD oxygen rates are typically designed with two parallel 50% air separation trains, each train comprising one vertical adsorber vessel, and one or two rotary-lobe type air blower/compressors providing combined sequential air pressurization-adsorption operation and vacuum desorption operation. The two trains typically share one common low-pressure oxygen product surge tank. As a required system rated TPD capacity increases, added parallel identical trains of VSA units are provided. For continuously operating facilities requiring greater than 350 TPD oxygen production ratings, the conventional VSA system employment of multiple parallel trains vertical adsorber vessels and rotary-lobe type blowers or compressors become marginally acceptable for several operating reasons. A sample review of U.S. Patents having variations of typical current art conventional VSA systems comprises:                (a) U.S. Pat. No. 5,658,371 describes a slightly modified conventional VSA air separation system of typical operating pressures, whereas U.S. Pat. No. 5,702,504 describes a complex variation of a conventional VSA air separation system process with overlapping operational steps of pressurization/adsorption/desorption/purging/pressure equalization. Such modifications or variations are typically directed to increase the productivity of the employed adsorbent and/or to decrease air separation power requirements.        (b) U.S. Pat. No. 5,114,440 describes a VSA process for the ‘enrichment of air’ with claims specifically addressed only to the employment of multiple layers of Ca Zeolite A within the three small parallel-connected conventional VSA vertical adsorber vessels, wherein each layer of Ca Zeolite A has different adsorption characteristics. Atmospheric air is drawn into a fan that then distributes a flow of slightly pressurized air into a manifold that connects to the bottom of each adsorber vertical vessel. For each adsorber vessel's adsorption step, air is emitted into each cylinder therein having an internal pressure of 1 bar or slightly less pressure that has been established from its equalization in pressure with the product gas manifold connected to all three adsorber vessels. Following the air's admittance into a given adsorber vessel and its subsequent 93% oxygen rich product flow out of the top of the vessel into the product gas manifold of 1 bar or less pressure, the product gas flows to another fan inlet that induces and propels a flow portion of the manifold's product gas at atmospheric pressure to an unspecified destination for its therein “oxygen-enrichment of air”. Without accounting for the fans energy, the vacuum pump 0.544 kWh/cubic meter oxygen production rate is equivalent to an exceptional high 364.8 kWh/ton production rate.        (c) U.S. Pat. No. 5,656,068 describes an improved vacuum pressure swing adsorption process and system having the objectives of higher efficiency and subsequent yielded reduced power costs (inventor contemplated to be about 20%) from those power costs of conventional current art VSA systems having ratings of up to 400 TPD. The VSA system employs two parallel adsorber trains, each train having two vertical adsorber vessels of inventors' stated preferred radial-flow design of vertical adsorber.        (e) U.S. Pat. No. 5,759,242 discloses the design of a vertical adsorber vessel having therein the internal means to direct gas flows radially through the molecular sieve adsorbent material contained within the vertical adsorber vessel. The ‘Background of the Art’ within U.S. Pat. No. 5,759,242, extensively describes the numerous operating shortcomings of conventional VSA vertical adsorber vessels having axial gas flows through the vertical beds of molecular sieve adsorbents. Listed earlier U.S. Patent art forms of vertical adsorber vessels having radial-flow adsorbent beds are therein disclosed and described, and the uniqueness of U.S. Pat. No. 5,759,242 features therein documented.        (f) U.S. Pat. No. 5,964,259 discloses the apparatus design and method of loading multiple molecular sieve adsorbents into the interior of the welded-closed vertical adsorber vessel therein designed to contain vertical radial-flow adsorbent beds as described in U.S. Pat. No. 5,759,242.        (g) U.S. Pat. Nos. 5,674,311, 5,538,544, and 6,334,889 respectively describe methods by which the conventional art VSA systems' (comprising vertical adsorbers and deep adsorbent beds) inherit problems of adsorbent bed temperature gradients, uneven gas flow distribution, and adsorbent bed fluidization can be reduced to improve adsorbent bed efficiencies.        
In summary, conventional VSA systems are marketed with required adsorber vessel forced-air supply pressures ranging from a low of 8 psig to a high of 12 psig (with 10 psig usually being the predominant average supply pressure at the air blower discharge connection) and with the predominant oxygen produced streams supplied to consumer facilities at 1.5 psig to 2.5 psig pressure.
Those skilled in the art will appreciate that the various approaches to VSA separation of gases, contained within the above example patents and other existing published art, predominantly share many common limitations that negatively impacts on their VSA system art's overall consistent gas product purity and economical power consumption. To overcome these common limitations and to satisfy the combined current operational requirements for large commercial, industrial, manufacturing, or power generation facilities (particularly those desiring to employ high-purity/high TPD oxygen capacity for pressurized oxy-fuel B.A.T. combustion systems) has led to the development of the herein described unique pure VSA separation system and apparatus invention having the following objectives:                1. It is a first objective to significantly reduce the electric power consumption required to produce a given desired gas product ton per day (TPD) production rate.        2. It is a second objective to provide an adsorption-desorption assembly that greatly reduces the molecular sieve adsorption bed depths associated with conventional VSA vertical adsorber vessels, thereby achieving reduced differential pressures across the molecular sieve bed and improved even distribution of gas flows throughout the molecular sieve bed.        3. It is a third objective to provide an adsorption-desorption assembly that greatly reduces the molecular sieve bed gas velocities as are employed within conventional VSA vertical vessel systems, thereby achieving significantly increased feedstock gases ‘residence time’ for gases to permeate into the porous structure of the molecular sieve adsorbent material.        4. It is a fourth objective to provide the means of eliminating conventional VSA deep molecular sieve adsorbent beds' operational temperature variance characteristics that negatively affect the beds' gas separation efficiencies.        5. It is a fifth objective to provide the available alternate apparatus and system means for air separation applications wherein the system has the ability to consistently maintain the rated oxygen purity and oxygen TPD rated production throughout all operating site annual ambient temperature conditions.        6. It is a sixth objective of the present pure VSA gas separation system and apparatus described herein that it can be adaptable to a manufacturer's or system fabricator's chosen selection of adsorbent molecular sieve materials, desired product gas production rate and purity, length and diameter dimensional configurations of adsorption-desorption assemblies, and the employment of the invention alternative features.        7. It is a seventh objective of the present pure VSA gas separation system and apparatus described herein (as configured for air separation production of a high-purity low-pressure, or alternate moderately high pressure oxygen product), that it can be capable of economically producing a facility production rating of up to 1500 TPD of high purity oxygen product as generated by a battery of parallel train described adsorption-desorption assemblies and a preferred quantity of two or more high efficiency compressors having low operating power consumptions and of currently available manufactured model configurations.        8. It is an eighth objective that adequate instrumentation and control devices be incorporated within the overall system to enable complete operational safety and product quality monitoring from the control of gas stream flows/temperatures/and pressures within PLC controlled sequenced adsorption and desorption operations.        9. It is a ninth objective of the present invention to provide a pure VSA gas separation system described herein that can be configured for the production of a high-purity low-pressure oxygen product that alternately can economically be increased in pressure by the addition of one or two stages of highly efficient gas pressure boosting compression.        10. It is a tenth objective of the invention that the herein described adsorption-desorption assembly can be fabricated and conventionally filled with the selected molecular sieve adsorbent at the time of manufacture, or the adsorption-desorption assembly be preferably configured at the time of manufacture to accept varied designs of inserted and removable molecular sieve adsorbent ‘cartridge-type’ sub-assemblies.        11. It is an eleventh objective of the present pure VSA gas separation system and apparatus described herein, when configured for air separation, that the system's predominant high purity oxygen product can be thereby acceptably employed for oxy-fuel combustion processes employing current best available technology (B.A.T.).        12. It is a twelfth objective that the pure VSA system can employ alternative or unconventional selections of gas compressors, vacuum pumps, adsorption-desorption assembly materials, heat exchange devices, instrumentation and control devices, and other auxiliary system sub-assemblies that utilize existing manufactured equipment components and materials of construction thereof which are not specifically designed for, nor applied to, the manufacture of current art VSA air separation systems.        13. It is a thirteenth objective that the employed adsorption-desorption assembly have the inherit design means that can accommodate the long-term operational employment of both present or later added future molecular sieve material adsorbents whose fragile structures can be incompatible with the cyclic pressure swings and weight bearing loads imposed by conventional vertical adsorber vessels having deep molecular sieve beds.        